Risk assessment and underlying mechanisms of pydiflumetofen resistance in Alternaria tenuissima and Alternaria alternata.

Potato early blight is a prevalent disease that significantly impacts potato cultivation. Pydiflumetofen, a new succinate dehydrogenase inhibitor developed by Syngenta, has been registered for use on multiple crops in China. However, its resistance risk and mechanisms in Alternaria tenuissima and Alternaria alternata are still poorly understood. The sensitivity of 404 A. tenuissima isolates and 41 A. alternata isolates to pydiflumetofen was evaluated, yielding mean EC of 0.257 ± 0.025 μg/mL and 0.083 ± 0.032 μg/mL, respectively. Through laboratory fungicide adaptation, three resistant mutants of A. tenuissima (RARQ2-1, RARQ2-2, and RARQ2-3) and one resistant mutant of A. alternata (RFQ1-1) were generated from two parental isolates. They exhibited relatively stable resistance, with resistance factors ranging from 37 to 173, and their compound fitness indexes either matched or exceeded that of their respective parental isolates. Pydiflumetofen demonstrated cross-resistance with boscalid, fluopyram, and cyclobutrifluram. Moreover, the resistant mutants had three unique substitutions in the SdhC proteins: AtSdhC and AtSdhC for A. tenuissima, and AaSdhC for A. alternata. The role of these substitutions in conferring pydiflumetofen resistance was validated via molecular docking, and they were detectable using allele-specific PCR. In summary, A. tenuissima and A. alternata exhibited high resistance risk to pydiflumetofen, with point mutations in the SdhC protein being the key contributors to this resistance.